According to the MPCA, “fine particulate matter is a complex mixture of very small liquid droplets or solid particles in the air. Major sources are cars, trucks, construction equipment, coal-fired power plants, wood burning, vegetation and livestock. Fine particulates are associated with increased hospitalizations and deaths due to respiratory and heart disease and can worsen the symptoms of asthma. People with respiratory or heart disease, the elderly and children are the groups most at risk.”

Sam Brungardt, public information officer with the MPCA, says the coming cold front is expected to push that air out, and reduce fine particulate matter to healthy levels by early afternoon.

The number of air quality alert days vary greatly from year to year, depending on the weather.

Cassie McMahon, Air Quality Research Analyst, emailed to say:

“Fine particle levels can increase at any time of year, but in Minnesota they are most often elevated in the winter months. Temperature inversions, low wind speeds, and moisture associated with warm (and often polluted) southerly winds contribute to increased particle levels across the state. In the winter of 2009/2010 a moderate El Nino contributed to abnormally stagnant weather conditions in Minnesota, which in turn resulted in a high number of fine particle events. However since that time, fine particle levels have actually been decreasing. The air alert this Saturday was the first fine particle driven air alert in the Twin Cities since January 2011.”